This invention relates generally to the field of plasma generation, and in particular, to apparatus and methods for generating persistent ionization plasmas.
Persistent ionization in air (PIA) plasmas are plasmas that are formed at atmospheric pressures and that persist for a finite time after termination of the power source. Large volume PIA plasmas have generated research interest because they are useful for simulating a phenomenon known as ball lightning, which is commonly observed in thunderstorms. In ball lightning, air and other gases are observed under certain conditions to have high levels of ionization for periods that are very long compared to the recombination times of the electrons. This is similar to the low loss electron phenomenon, which is readily observed in PIA experiments in the laboratory.
In ball lightning, electron recombination times in air, hastened by electron attachment to oxygen and water, are on the order of 10 microseconds. But appreciable levels of ionization appear to precede the main lightning discharge by 10 msec and persist for periods of 10 msec or longer afterwards. This is called the stepped leader phenomenon. This phenomenon and the unexplained interval between discharges is commonly observed in lightning storms.
Several theoretical models have been proposed in the past for ball lightning. These models suggest the involvement of RF radiation. An early theory explained ball lightning as an evacuated microwave resonant cavity surrounded by a layer of plasma. Another theory proposed that vorticity can play a part. A recent theory describes ball lightning as an electromagnetic knot, with tangled magnetic fluxes. The electromagnetic knot model predicted an expansion of the plasma as it cools, in the limit of infinite conductivity.
The process of plasma formation in air by microwaves has also been extensively investigated, both experimentally and theoretically. As a result, it is known that the formation of plasmas in air, O2, and N2 are fairly similar. Breakdown is achieved at lower field strengths with lower frequencies: approximately 1000 V/cm will achieve breakdown in room air at 0.992 GHz, whereas approximately 3000 V/cm is required at 9.4 GHz.
A number of researchers have produced PIA plasmas using high-frequency electromagnetic fields at atmospheric pressure to simulate ball lightning. Kapitza originally formulated a theory that ball lightning forms from RF waves in the atmosphere. Tesla made the earliest report of an artificial creation of ball lightning. Later, Powell and Finkelstein succeeded in making spherical discharges that would separate from the electrodes where they formed. They used 75 MHz RF at 20 kW and a 15-cm-diameter Pyrex tube to form the plasmas. Powell and Finkelstein found that the large volume plasmas produced in those experiments persisted for as much as 0.5 seconds after termination of the ionizing radiation.
In more recent experiments, researchers used a 1-5 kW 2.45-GHz power source to drive a resonant cavity, but did not restrict the physical extent of the plasmas formed. The researchers created large air discharges in the resonant cavity. These discharges were often augmented by ordinary combustion. Other researchers have used helium gas as a plasma medium at atmospheric pressure.
In previous experiments for creating PIA plasmas, high-power sources, resonant cavities, or specialized gases were needed in order to create large plasmas at atmospheric pressure. No method or device currently exists for creating PIA plasmas with commercially available equipment, such as commonly available gases and power sources. Further, previous research efforts have not succeeded in measuring the properties of the created plasmas. Accordingly, there currently exists a need for apparatus and methods for creating PIA plasmas efficiently and economically, and for measuring the properties of the created PIA plasmas.
It is a principal object of the present invention to efficiently and economically generate steady state plasmas that are formed at atmospheric pressure and that persist for a finite time after termination of the power source (i.e. persistent ionization in air, PIA, plasmas). It is another principal object of the invention to create a steady state plasma where the electrons in the plasma have poor thermal transfer to the neutral atoms, thereby keeping the ambient gas temperature low. It is yet another principle object of the invention to provide apparatus and methods for measuring the properties of the generated PIA plasmas, such as plasma lifetimes after termination of the driving electric fields, and densities of electrons and ions.
It is yet another object of the invention to create a large volume steady state plasma that persist for a time after creation, without the use of discharge electrodes. It is another object of the invention to use such plasmas as shields against microwave beams. It is another object of the invention to use such plasmas to reduce the aerodynamic drag of aircraft. It is another object of the invention to use such plasmas to generate high efficiency illumination. It is another object of the invention to use such plasmas as an excited source for a gas laser. It is another object of the invention to use such plasmas to produce ozone for toxic gas abatement.
Accordingly, the present invention features a persistent ionization plasma generator that includes a RF cavity that is in fluid communication with a source of ionizing gas. The cavity can be substantially at atmospheric pressure. An RF power source that generates an RF electric field is electromagetically coupled to the RF cavity. The RF power source can operate at 2.45 GHz or at 915 MHz. An ultraviolet light source is positioned in optical communication to the cavity.
The ultraviolet light source can be a spark plug or a laser. A nozzle that is coupled to the source of ionizing gas can be positioned to inject the ionizing gas into the cavity proximate to the ultraviolet light source. An antenna is positioned within the cavity adjacent to the ultraviolet light source. A chamber for confining the plasma can be positioned in the cavity around the antenna and the ultraviolet light source. The chamber can be positioned at an angle relative to the cavity in order to cause a vortex flow of the ionizing gas in the chamber. A plasma is formed in the cavity that persists for a time after termination of the RF electric field.
The present invention also features a method of generating a persistent ionization plasma. The method includes injecting an ionizing gas into a RF cavity. The ionizing gas can be mixed with ambient air in the cavity. A vortex flow of the ionizing gas can be formed in the cavity. An RF electric field is electromagnetically coupled to the cavity. An antenna is provided that assists in the ignition of a plasma. Ultraviolet radiation is then optically coupled into the cavity in order to cause ignition of a plasma.
The RF electric field is terminated and the plasma persists for a time after termination, which can be greater than 1 ms. The plasma can persist for a time after termination of the RF electric field because electron motion in the plasma resulting from collisions between free electrons and electrons bounded to neutrals is decoupled.
The present invention also features a method for reducing aerodynamic drag of an aircraft. The method includes positioning an antenna on a surface of an aircraft. A RF electric field is electromagnetically coupled to the surface of the aircraft proximate to the antenna. Ultraviolet radiation is also optically coupled to the surface of the aircraft proximate to the antenna in order to cause ignition of a plasma. The RF electric field is terminated and the plasma persists for a time after termination. The electrons in the plasma that persists for a time after termination of the RF electric field have reduced thermal transfer to neutral atoms and, therefore, reduce aerodynamic drag on surface of the aircraft.
The present invention also features a method of exciting a gas laser. The method includes injecting an ionizing gas into a laser cavity. A vortex flow of the ionizing gas can be induced in the cavity. A RF electric field is electromagnetically coupled to the laser cavity. An antenna is provided in the laser cavity that assists in the ignition of a plasma. A pump laser beam is optically coupled into the laser cavity in order to cause ignition of a plasma. The RF electric field is terminated and the plasma persists for a time after termination. The plasma causes laser oscillations in the laser cavity.
The present invention also features a method of toxic gas abatement. The method includes injecting an ionizing gas and a toxic gas into a RF cavity. A vortex flow of the ionizing gas can be induced in the cavity. A RF electric field is electromagnetically coupled to the cavity. An antenna is provided in the laser cavity that assists in the ignition of a plasma. Ultraviolet radiation is then optically coupled into the cavity in order to cause ignition of a plasma. The RF electric field is terminated and the plasma persists for a time after termination. The plasma abates the toxic gas.
In addition, the present invention features a method of characterizing a persistent ionization plasma. The method includes forming a RF electric field generated plasma in a cavity. An illuminator is positioned in the cavity that radiates optical radiation when exposed to RF electric field. The optical radiation generated by the illuminator and by the plasma is recorded by a recording device. The time period during which the plasma persists after termination of the RF electric field is determined by counting frames that record the radiation being generated by the plasma while substantially no radiation is being generated by the illuminator. The method can include the step of inserting a Langmuir probe into the plasma to measure density and temperature of electrons in the plasmas during the time period. The method can also include the step of inserting a loop probe into the plasma to measure the electric field in the plasmas during the time period.